Electrochemical Concentration of Lanthanide and Actinide Elements

ABSTRACT

A carbon paste electrode is modified with a chemical agent that is selective for a plurality of lanthanides and actinides (f-series) elements. The modified carbon paste electrode selectively has different voltages applied thereto where a first voltage is used to cause the deposition of one or more lanthanides or actinides from an industrial or environmental sample onto the electrode, and, subsequent to removal of the electrode from the sample and insertion into a second sample where concentration of lanthanides or actinides is preferred, a second voltage is used to cause the deposited lanthanides and/or actinides to be discharged from the electrode for concentration into the second sample.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional 61/434,919 filedJan. 21, 2010, and the complete contents of that application isincorporated herein by reference.

This invention was made with government support under contract numbersECCS-0833548 and DN-077-ARI-03302 provided respectively by theDepartment of Homeland Security and the National Science Foundation. Thegovernment has certain rights in the invention.

BACKGROUND

It has been known since the 1950's that separation, pre-concentration,and purification of f-elements (those elements in the lanthanide andactinide series of the periodic table) could be achieved throughformation of mercury amalgams.^(1,2) Early techniques used mercury poolsas the cathode and more recent work has focused on thin mercury films todetect or pre-concentrate f-elements.³ Due to the inherent toxicity ofmercury, a stronger focus has been on developing mercury-freetechniques.

Since its first reporting in 1958 by Ralph N. Adams⁴, carbon paste (CP)electrodes have become a widely used electrode material inelectrochemical research⁵. Many factors have lead to their popularity:low ohmic resistance⁶, large potential window⁵, and ease ofmodification⁷, to name a few. A quick review of recent literatureindicates that CP or modified carbon paste (MCP) electrodes areapplicable to aqueous^(8,9) and non-aqueous^(10,11) matrices indetermination of organic^(12,13) and inorganic^(14,15) elements andcompounds. Examples of manufacturing CP and MCP electrodes used aselectrochemical sensors and other applications can be found in U.S. Pat.No. 7,968,191 to Hampden-Smith, U.S. Pat. No. 7,901,555 to Jiang, andU.S. Pat. No. 6,828,358 to Morrison, each of which is hereinincorporated by reference.

While applications of CP and MCP electrodes are numerous, relativelylittle is reported in the area of f-elements. Li et al.¹⁶ developed anovel MCP electrode with alizarin used as the complexant modifier. TheMCP electrode responded well for the middle to heavy lanthanides with alimit of detection (LOD) of 10⁻¹⁰ M for Ho³⁺ in an acetate buffer.Linear sweep voltammetry (−0.2V to 0.8V vs. SCE) was applied after a60-120 second pre-concentration period at −0.2 V. A linear,concentration-dependent signal was obtained for the range 10⁻¹⁰ to 10⁻⁷M, and concentrations of the heavier lanthanides in a dissolved, castiron sample were quantitatively determined using this electrode. Li'ssubsequent work¹⁷ focused on Ce³⁺ using the same electrode and similarsolution conditions described previously. After optimization, thealizarin MCP electrode exhibited a LOD for Ce³⁺ of 10⁻⁹ M and a linearresponse range of 10⁻⁹ to 10⁻⁷ M.

Ganjali et al.¹⁸ developed an ion selective electrode (ISE) for Ho³⁺utilizing a MCP electrode containing multi-walled carbon nanotubes,nanosilica and the ionophoreN′-(2-hydroxybenzylidene)furan-2-carbohydrazide in addition to graphite.The MCP electrode had a detection limit of 10⁻⁸ M for Ho³⁺ and a linearresponse range from 10⁻⁷ to 10⁻² M. Additionally, a single conditionedelectrode showed a reproducible stable response to standard solutionsfor up to two months. Continuing the work of Ganjali et al., Norouzi etal.¹⁹ developed an Er³⁺ ISE with the same basic components in the MCPelectrode except the ionophore was changed toN′-(2-hydroxy-1,2-diphenylethylidene)benzohydrazide. Response of theelectrode to Er³⁺ was similar to that demonstrated by Ganjali et al.with a LOD of 10⁻⁸ M and a linear range of 10⁻⁷ to 10⁻² M.

A critical limitation of these systems is their lack of generalapplicability to the f-elements as a group of cations. There is a needin the art for systems and methods capable of simultaneouslyconcentrating the full range of f-elements.

SUMMARY

In an embodiment of the invention, MCP electrodes are made operable toselectively concentrate f-elements from a dilute solution andsubsequently release the concentrated f-elements for follow-onseparation and detection. This embodiment is a mercury free process,and, although not previously recognized in the art, CP and MCPelectrodes are particularly suited to perform this type of concentrationof f-elements.^(20,21) The terms CP and MCP electrodes are at times usedinterchangeably in this description, however, it will be understood thatthe invention is focused on a carbon paste electrode, sometimes referredto as a conducting ink/paste electrode which can be formed from carbonblack, graphite, carbon powder, carbon flake, carbon nanotubes, etc.,where the carbon is modified to permit selective binding of f-serieselements (as opposed to an unmodified carbon paste electrode of a carbonpaste electrode which cannot selectively bind f-series elements).

In another embodiment of the invention, an MCP electrode is providedwherein the modifying agent is a chemical agent with a selective bindingaffinity for f-elements. Exemplary modifying agents for the MCPelectrode which permit selective binding of f-elements are selected fromat least one of: 1,2-dihydroxybenzene-3,5-disulfonic acid,2-hydroxyisobutyric acid, trimetaphosphoric acid,trans-1,2-cyclohexylenedinitrilotetraacetic acid,2-hydroxy-2-methylpropanoic acid, iminodiacetic acid, nitrilotriaceticacid, ethylenedinitrilotetraacetic acid, diethylenetriamine-pentaceticacid,2,2′,2″,2′″-(1,4,7,10tetraazacyclododecane-1,4,7,10-tetrayl)tetraaceticacid, N-(2-carboxyphenyl)iminodiacetic acid, dihydroxycyclobutenedione,3,6,9,12-tetrakis(carboxymethyl)-3,6,9,12-tetraazatetradecane-1,14-dioicacid and derivatives of these compounds. Derivatives comprise compoundswith identical f-element ligation sites and a similar structural motif(e.g. 2-hydroxyisobutyric acid and 2-hydroxybuteric acid).

In yet another embodiment of the invention, a method for concentratingf-elements is provided. This method includes contacting a modifiedcarbon paste (MCP) electrode, wherein the modifying agent is a chemicalagent with a selective binding affinity for f-elements, with anenvironmental or industrial sample containing at least one f-element;applying a voltage to the electrode suitable to deposit f-elements ontothe electrode; moving the electrode to a second sample of a volume lessthan the first sample and; applying a second voltage suitable to releasef-elements from the electrode thereby concentrating the f-elements inthe second volume (because the second volume is smaller than the first,the f-elements are more concentrated in the second volume; however, itwill be recognized that this method could also be used to simplytransfer f-elements from one volume to another without limiting the sizeof the second volume). In variations on this method, the electrode canbe cycled between environmental or industrial samples and a secondsample applying the first voltage within the environmental or industrialsample and the second voltage within the second sample.

The MCP electrodes comprise a conductive wire, well or surface with apre-mixed suspension of a conductive graphite and chemical modifyingagent with a binding liquid are disposed thereon. See [Joseph Wang,Balashaheb K. Deshmukh, Mojtaba Bonakdar, Solvent extraction studieswith carbon paste electrodes, Journal of Electroanalytical Chemistry andInterfacial Electrochemistry, Volume 194, Issue 2, 25 Oct. 1985, Pages339-353.] for examples of different graphite-binding liquid combinationsfor CP electrodes which may be employed in the practice of thisinvention.

Ease of modification is one of the most valuable features of CPelectrodes.⁵ This is due to the well-developed surface of CP which has ahigh adsorptivity.²² Modification of CP electrodes can be achievedthrough a multitude of methods. A few of these methods are: chemicalpre-treatment where the carbon is soaked in the modifier and thenevaporated to dryness before being prepared as an electrode;²³ in situmodification where the modifier adsorbs to the surface of plain CPelectrode thus allowing for determination of analyte in the solution;²⁴dissolution in the binding liquid which is typically achieved throughthe use of an ion-exchange resin;²⁵ or direct mixing of dry modifiersinto the paste through mechanical means which is believed to be the mostfrequently used method.²²

DESCRIPTION OF THE DRAWING FIGURES

FIGS. 1A-B: Panel A is a CV scan showing the electrochemical response ofa CP electrode in 0.1 M LiCl. Panel B is a HIBA-CP, solid line,electrode response in the same 0.1 M LiCl solution. For comparison, theCP electrode response is plotted in Panel B as the dashed line. Scanrate was 100 mV/s, pH 3.5 and potential reported vs. Ag/AgCl. Scanbegins and ends at 0.8 V for both voltammograms.

FIG. 2: A family of scans for a single HIBA-CP electrode in 0.1M LiCl,pH 3.5. Scan rate was varied from 1 to 500 mV/s. The ratio ofi_(c)/i_(a) was 1.12±0.03 suggesting chemical reversibility. Generalwave shape did not follow the Nernst equation indicating electrochemicalirreversibility.

FIG. 3: Background subtracted CV for 0.1 M HIBA on a 3 mm Pt workingelectrode. Background electrolyte was 0.1 M LiCl, pH 3.5, scan rate 100mV/s. Potential is reported vs. Ag/AgCl. Nernstian and faradaiccomparison to 0.1 mM K₃[FeCN₆] on the same electrode indicates anelectron transfer of 1.2±0.2 electrons.

FIG. 4: Anson Plot for CP and HIBA-CP electrode response to La³⁺. Solidline represents the response of HIBA-CP to a mixture containing 0.1MLiCl and 1×10⁻⁵ M La³⁺. Dashed line is CP response to an identicalmixture of LiCl and La³⁺. The CP line intersects on the x-axisindicating no sorption phenomenon. The HIBA-CP intersection above thex-axis is indicative of a sorption phenomenon.

FIG. 5: Results of 1 second DPSC experiments with a HIBA-CP electrode.Signal intensity was measured at 500 msec and background subtracted. Theconcentration range for La³⁺ covered from 10⁻⁷ M to 10⁻³ M, n=3 for eachdatum point. Errors bars are ±1 σSD. The shape of concentrationdependent response is indicative of a sorption isotherm.

FIGS. 6A-Q: ICPMS results for pre-concentration of select f-elements.LOD was determined from 7 blank runs+3 σSD. Closed circles represent theresults from calibration standards (1 ppb-1 ppt). Open trianglesrepresent the pre-concentration results of 5 ppq with the HIBA-CPelectrode, n=5. Open diamonds represent pre-concentration results of 5ppq with a CP electrode. Open squares are the results of 5 ppq solutionswith no pre-concentration. Errors bars are 1 σSD most of which arewithin the dimensions of the symbols. Y-axis for all graphs areexpressed in terms of counts per second.

FIGS. 7A-C: Schematic representations of a modified carbon pasteelectrode suitable for use in device operable to electrochemicallytransfer f-series elements from a first medium to a second medium.

FIG. 8: Process overview for a device operable to electrochemicallytransfer f-series elements from a first medium to a second medium.

DETAILED DESCRIPTION

The description below shows the fabrication and testing of certainexemplary electrodes and methods according to the invention. It will berecognized by those skilled in the art that the electrodes, thematerials used for their fabrication, and methods of use can be variedwithin the spirit and scope of the appended claims.

Methods and systems for the electrochemical transfer of f-series elementconstituents present in a first medium to a second medium are describedherein. A modified paste electrode can be utilized to accumulatef-series elements within the paste when the voltage applied to theelectrode is held at a first voltage and subsequently released from thepaste when the voltage applied to the electrode is held at a secondvoltage. The ability to accumulate f-series elements enables a range ofapplications wherein said elements can be transferred between media. Inparticular embodiments the transfer of said elements can serve toconcentrate the elements (e.g. when the volume of the second medium isless than the volume of the first medium or where the elements fromplurality of first media are deposited in a common second medium).

An exemplary modified carbon paste electrode is schematically depictedin FIGS. 7AC. In reference to FIG. 7A a typical implementation for amodified carbon paste electrode comprises a paste 703 on the surface(portion of the component in diffusive communication with the media) ofa conductive element 702 encased within an insulating housing 701, and aconnector 704 operable to connect the conductive element 702 to anexternal voltage source. The implementation depicted in FIG. 7A providesan example wherein the surface or tip of the conductive element 702 iscoplanar with the tip of the insulating housing 701. Other embodimentsmay comprise configurations where the tip of conductive element 702 andpaste 703 are not coplanar. In particular embodiments the paste 703 mayalso be present in a void in the insulating housing 701 as depicted inFIG. 7B. In such implementations the surface of the conductive elementmay not extend to the tip of the insulating housing. FIG. 7B may alsocomprise an embodiment wherein the conductive element 702 is a porousmaterial and the paste has been distributed into the pores of theconductive element 702. In yet further embodiments, the tip of theconductive element 702 may extend further from the tip of the insulatinghousing 701 as depicted in FIG. 7C. Common to all configurations is thatthe paste 703 is applied to the portion of the conductive element 702that is in diffusive communication with the external medium (not shown).

The insulating housing 701 may comprise any material that does notconduct electrical current. Examples include, but are not limited toGlasses, Ceramics, polyethylene, crosslinked polyethylene (eitherthrough electron beam processing or chemical crosslinking), polyvinylchloride (PVC), Kapton, rubber-like polymers, oil impregnated paper,Teflon, silicone, and/or modified ethylene tetrafluoroethylene (ETFE).The conductive element 702 may comprise a material that can conductelectrical current. Examples include, but are not limited to platinum,gold, silver, glassy carbon, brass, copper, graphite, porous graphite,and/or molybdenum, and combinations thereof.

The paste 703 applied to the portion of the conductive element 702 thatis in diffusive communication with the external medium is composed ofthree general components: a binder, a conductive component, and amodifier. The binder serves to adhere the paste 703 to the surface ofthe conductive element 702 and provide a fluid like medium to uniformlydisperse the conductive component and modifier within the binder.Traditional binders comprise organic liquids which link mechanically theconductive component and modifier. However, besides this main function,the binder as the second main moiety of carbon paste co-determines itsproperties. Typical parameters required for binders are: i) chemicalinertness and electroinactivity, ii) high viscosity and low volatility,iii) minimal solubility in aqueous solutions, and iv) immiscibility withorganic solvents. Example binding agents (binders) used for preparationof carbon pastes include, but are not limited to; mineral (paraffin)oils; namely, i) Nujol or a similar trade-mark product and solvent forspectroscopy ii) Uvasol iii) aliphatic and aromatic hydrocarbons,including their iv) halogenated derivatives, as well as v) silicone oilsand greases, or nearly solid silicone rubbers.

The conductive component within the paste typically comprises acarbonaceous material. In particular embodiments powdered carbon(graphite) as the conductive component within the paste provides forproper function of an electrode or a sensor in electrochemicalmeasurements. Suitable carbonaceous materials should obey the followingcriteria: i) particle size in micrometers, ii) uniform distribution ofthe particles, iii) high chemical purity, and iv) low adsorptioncapabilities. Naturally, the type and quality of graphite used, as wellas its overall amount in the carbon paste mixture, are reflected in alltypical properties of the respective mixture. A typical carbon powdercomprises spectroscopic graphite with particles in the low micrometricscale (typically, 5-20 mm). Alternatives to graphite include but are notlimited to i) soot and charcoal, ii) acetylene black, iii) glassy carbonpowders with globular particles, iv) pulverized diamond of both naturaland synthetic origin, v) template carbon, vi) porous carbon foam, andvii) carbon microspheres viii) fullerenes, ix) carbon nanofibers orvarious types of x) carbon nanotubes. In general the conductivecomponent is present in a concentration of between 5 g/ml binder and 0.2g/ml binder within the paste.

The modifier in the paste generally comprises an organic compound thatcontains a (or a plurality of) functional group that demonstrates apreference for ligating to f-series elements. In the preferredembodiment the organic compound has an affinity for a range of f-serieselements. In yet further embodiments a plurality of organic compoundsmay be incorporated as modifiers wherein each organic compound providesa preference for binding a distinct subset of elements within the tof-series elements. Examples of organic compounds include but are notlimited to, 1,2-dihydroxybenzene-3,5-disulfonic acid,2-hydroxyisobutyric acid, trimetaphosphoric acid,trans-1,2-cyclohexylenedinitrilotetraacetic acid,2-hydroxy-2-methylpropanoic acid, iminodiacetic acid, nitrilotriaceticacid, ethylenedinitrilotetraacetic acid, diethylenetriamine-pentaceticacid,2,2′,2″,2′″-(1,4,7,10tetraazacyclododecane-1,4,7,10-tetrayl)tetraaceticacid, N-(2-carboxyphenyl)iminodiacetic acid, dihydroxycyclobutenedione,3,6,9,12-tetrakis(carboxymethyl)-3,6,9,12-tetraazatetradecane-1,14-dioicacid and derivatives of these compounds. Derivatives comprise compoundswith identical f-element ligation sites and a similar structural motif(e.g. 2-hydroxyisobutyric acid and 2-hydroxybuteric acid). In generalthe modifier (or modifiers) is present in a concentration of betweenabout 0.1 mM and 20 mM within the binder. The concentration of modifiercan be tailored to the f-element content within the intended operationalmedia.

FIG. 8 provides an operational process for accumulating f-elementspresent in a medium and transferring the f-elements to a second medium.A modified paste electrode can be utilized to accumulate f-serieselements within the paste when the voltage applied to the electrode isheld at a first voltage within a first medium. Implementations whereinthe first medium is an aqueous medium the applied voltage required isabout −0.05 to −0.05 V (vs. Ag/AgCl). The duration of time necessary foraccumulation will vary with the specific implementation. Once thef-elements are accumulated, the electrode can be transferred from thefirst medium to a second medium. An optional cleaning step may be addedduring this transfer; here the cleaning may involve a physical orchemical cleansing of the body of the electrode. Once the electrode isplaced in the second medium, a second voltage is applied to release thef-elements accumulated in the paste. Implementations wherein the secondmedium is an aqueous medium the applied voltage required is about 0.1 to0.09 V (vs. Ag/AgCl). The specific voltage applied should be dependentupon both the operational media and the organic compounds present withinthe paste. More specifically, the accumulation voltage should be avoltage wherein the organic group is reduced and the release orstripping voltage should be at a voltage wherein the organic group is init's electrochemical ground state (e.g. not oxidized or reduced). Theprocess of accumulation and stripping may be repeated or cycled betweenmedia.

In a specific particular embodiment, Lanthanide cations in solution canbe rapidly sequestered onto and subsequently removed from a modifiedcarbon paste electrode by cycling the voltage of the electrode. Theelectrode comprises a paste produced by mechanically mixing 5 grams ofAcheson 38 carbon with 3 milliliters of paraffin oil. Prior to formingthe paste with the carbon, the paraffin oil is modified by mixingapproximately 5 millimoles (300 milligrams) of alpha-hydroxyisobutyricacid (HIBA) into the 3 milliliters of paraffin oil. The required rangeof concentration of HIBA in paraffin oil is 0.1-20 mol/L. This paste isthen applied to the end of a Teflon electrode body. As a group of metalcations, the lanthanides accumulate from a solution of 0.1 M LiCl ontothe carbon paste surface within 30 second when a voltage of −0.4 V (vs.Ag/AgCl) is applied. The sorbed lanthanide cations can then bequantitatively stripped off the electrode surface into a differentsolution by applying an oxidative step of +0.8 V (vs. Ag/AgCl).

EXAMPLES

Materials: Reagent grade graphite, LiCl, paraffin oil and2-hydroxyisobutyric acid (HIBA) were used as received, from FisherScientific, (Waltham Mass. USA, www.fishersci.com). For themulti-element analysis, a stock solution containing 10 ppm of analytes(Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Th)in 2% HNO₃ was purchased from High-Purity Standards, (Charleston S.C.USA, www.highpuritystandards.com). This solution was used as received.

Electrode Fabrication: Procedures for preparation of the CP electrodewere modified from Adams.²⁶ In brief, 5 grams of Acheson 38 gradegraphite was mechanically mixed with 3 mL of paraffin oil using a glassmortar and pestle to produce a thick, uniform paste. To prepare theHIBA-CP electrode, 5 mmoles of solid HIBA were added to 3 mL of paraffinoil and mixed until the slurry was homogenous. Then five grams of carbonwere added and mixed to form the paste. Procedures for smoothing andrenewing the electrode followed those of Adams, which is hereinincorporated by reference.²⁶

Electrochemical Procedures: Electrochemical measurements were made usinga Model 100B potentiostat, 15 mL Teflon™ electrochemical cell, and 3 mmCP Teflon™ electrode body purchased from Bioanalytical Systems Inc.(West Lafayette Ind. USA, www.basinc.com). The 15 mL Teflon™electrochemical cell was constructed and setup similarly to Schumacheret al.³ The reference and auxiliary electrodes were Ag/AgCl and Pt wire,respectively. Solutions within the electrochemical cell were purged withpurified Ar prior to conducting an experiment. Cyclic voltammetric (CV)experiments were typically scanned at a rate of 100 mV/s starting at+0.8 V to −0.4 V. Double potential step chronoamperometric (DPSC) andchronocoulometric (CC) experiments were stepped from +0.8 V to −0.4 Vand back to +0.8 V vs. Ag/AgCl. The background electrolyte was 0.1 MLiCl that was pH adjusted using 2% HNO₃. Pre-concentration and strippingexperiments followed the procedures outlined in Wang^(27,28) withmodifications. After a deposition step of 30 sec, the CP or HIBA-CPelectrode was removed from the cell, wiped with a Kim-Wipe on theinsulating shroud and transferred to a separate vial containing 2 mL of2% HNO₃. A stripping step from −0.4V to +0.8V vs. Ag/AgCl for 30 sec,was performed and the solution analyzed by ICPMS. A typical experimentinvolving conditioning the CP or HIBA-CP electrode, pre-concentration ofthe trivalent f-element and stripping into 2% HNO₃ requiredapproximately five minutes. The cell was cleaned between experimentsfollowing the procedures of Schumacher et al.³

ICPMS Procedures: Inductively Coupled Plasma Mass Spectrometry (ICPMS)measurements were performed on an Agilent 7500 ICPMS utilizing aninternal indium and rare earth standard and scanned in the positivemode. Prior to analyzing any samples, the instrument was calibrated witha set of prepared standards in 2% HNO₃ and plain 2% HNO₃ was used as theblank to correct for background.

Analyses of electroanalytical and ICPMS data were performed usingGraphPad Prism version 5.02 for Windows, (GraphPad Software, San DiegoCalif. USA, www.graphpad.com).

FIG. 1A shows the electrochemical response for a CP electrode to 0.1MLiCl at pH 3.5. While this voltammogram is only the response tobackground electrolyte, a series of scans were performed in varioussolutions containing HIBA and/or f-elements to determine if CP exhibitedany electrochemical response. In these cases, the CP electrode showed noelectrochemical response to dissolved f-elements.

FIG. 1B shows the electrochemical response for a HIBA-CP electrode to0.1 M LiCl at pH 3.5; the pKa for HIBA is 3.7.²⁹ For this voltammogram,the scan rate was 100 mV/s and started at +0.8 V. A large reduction andoxidation signal was observed for the HIBA-CP electrode. Each newHIBA-CP electrode typically required two to three conditioning scans toachieve a stable (exceeding three hours) electrochemical response.Variability of peak intensity between electrodes was less than 10%,which falls within the range expected for MCP electrodes.²⁷Additionally, the cathodic and anodic peaks varied less than 5% betweenelectrodes for a given scan rate. This demonstrates that onceconditioned, the HIBA-CP electrode yields a stable and reproducibleresponse.

Watanabe et al.³⁰ reported that f-elements will adsorb to carbonaceousmaterial in acidic environments. Our result for the CP electrode is notin disagreement with Watanabe et al. as the contact time for their studywas much longer (3-4 hours)³⁰ than the time period used in this study(1-2 min).

FIG. 2 shows a series of CVs for a single HIBA-CP electrode in 0.1 MLiCl where the scan rate was varied from 1 to 500 mV/s. The measuredratio of cathodic (i_(c)) and anodic (i_(a)) peak intensities wasconstant across scan rates at 1.12±0.03 (n=3) suggesting theelectrochemical response from the HIBA-CP is chemically reversible. Theplots of scan rate, υ, versus i_(p) and υ^(1/2) versus i_(p) wereinconclusive as to whether the observed results represent an adsorptionor diffusion phenomenon. This result is not surprising given the rangeof υ used. The shapes of the voltammograms did not follow the Nernstequation, suggesting that electrochemical charge transfer isirreversible.

To further evaluate diffusion vs. adsorption, chronoamperometricanalysis of the voltammetric wave shapes were conducted using theCottrell equation:

$\begin{matrix}{i = \frac{{nFAC}\sqrt{D}}{\sqrt{\pi \; t}}} & (1)\end{matrix}$

where i=current (amps), n=number of electrons, F=Faraday constant(96,485 C/mol), A=area of the electrode (cm²), C=initial concentrationof the analyte (mol/cm³), D=diffusion coefficient for the species(cm²/s), and t=time (s) was used to evaluate the waveforms. A plot oft^(−1/2) vs. i deviated from linearity based on time of exposure to theanalyte, suggesting other processes were either occurring at the surfaceof the electrode or impeding diffusion to the electrode surface.^(31,32)A series of experiments were performed at different values of pH and nopH effects were observed, suggesting that the observed phenomenon isoccurring on the surface of the electrode and not a direct result ofsolution conditions.

To determine the number of electrons transferred per mole of HIBA, asolution containing 0.1 mM K₃[FeCN₆] and 0.1 M LiCl, which has a known 1e-transfer, [Fe(CN)₆]³⁻+e⁻

[Fe(CN)₆]⁴⁻, was analyzed by CV on a 3 mm Pt electrode to determine thei_(p) and integrated voltammetric wave area. A separate solutioncontaining 1 mM HIBA and 0.1 M LiCl was analyzed by CV on the same Ptelectrode. (FIG. 3) Comparison of the ratios of magnitude of i_(p) andintegrated voltammetric wave areas for HIBA to K₃[FeCN₆] indicated anelectron transfer of 1.2±0.2 electrons per mole of HIBA.

Kvaratskhelia and Kvaratskhelia³³ examined the voltammetric responses ofhydroxycarboxylic acids in aqueous solutions using various solidelectrodes. Their E_(1/2) values of the observed waves on Pt in 0.1 MNaClO₄ occurred in the range of −0.47 to −0.49 V vs. a saturated calomelelectrode. Our voltammetric response for HIBA is in agreement with theirobserved results.

In a MCP electrode study involving complexes between rare earths andalizarin, Li et al.¹⁶ reported a 2 e⁻ charge transfer irreversibleprocess for alizarin that was not pH dependent. They point out that mostelectrode processes of organic compounds involve proton ion transfersthus a pH dependence is expected; however, in the case of alizarin thiswas not observed. Our characterization of HIBA, a 1 e⁻ irreversibleprocess with no pH dependence, agrees with the characterization reportedby Li et al.¹⁶ for alizarin.

FIG. 4 shows the resultant Anson plot³⁴ for a three secondchronocoulometric experiment using both a HIBA-CP and CP electrode in0.1 M LiCl and 1×10⁻⁵ M La³⁺. The reduction lines are plotted as t^(1/2)vs. Q and the oxidation lines are plotted as θ vs. Q, whereθ=[τ^(1/2)+(t−τ)^(1/2)−t^(1/2)] as defined by Anson.³⁴ For HIBA-CP thereduction and oxidation lines exhibit different slopes with anintersection above the x-axis. The CP reduction and oxidation lines havenearly identical absolute values of their slopes and the lines intersecton the x-axis. One second and five second chronocoulometric experimentswere also performed with nearly identical results to the three secondexperiment (data not shown). This range of timescales were chosen tominimize contributions from convective mass transport, which occurs atsolid electrodes at time periods greater than 5 seconds.²⁶ According toAnson, intersection above the x-axis represents the total coulombs ofadsorbed reactant because this analysis of chronocoulometric dataeffectively removes any contribution due to double layercharging.^(34,35) Using the intersection value, 1.1±0.3×10⁻⁶ C, withFaraday's Law; the total number of moles accumulated equals3.6±0.7×10⁻¹², n=3.

FIG. 5 is the results of chronoamperometric experiments with a HIBA-CPelectrode in varying solution concentrations of La³⁺ (10⁻⁷ M to 10⁻³ M)with 0.1 M LiCl as the background electrolyte at pH 3.5. The data pointswere obtained by running 1 second chronoamperometric experiments (anexperimentally convenient time interval) and measuring a backgroundcorrected i value at 500 ms. This time span was chosen because itexcludes distortion due to charging current and minimizes contributionsdue to convective mass transport.³⁶ A new HIBA-CP electrode was used foreach change in the concentration of La³⁺ and each point represents theaverage of triplicate runs. The shape of the graph shows a concentrationdependent signal suggesting a sorption phenomenon.

FIG. 6 shows representative results from the pre-concentrationexperiments. The multi-element standard used contained all thelanthanides, minus promethium, plus scandium, yttrium, and thorium. Thefour graphs represent the range of responses of the lanthanides andshows that the HIBA-CP electrode will pre-concentrate above the limit ofdetection (LOD) for the ICPMS while the CP electrode does notpre-concentrate under the same conditions as the HIBA-CP electrode. Thecounts for all the lanthanides were totaled and applied to a calibrationcurve to determine total moles accumulated, 3.0±0.6×10⁻¹², n=3.Comparing this number to the total moles adsorbed via the Anson plot inFIG. 5, 3.6±0.7×10⁻¹² we find good agreement indicating that the HIBA-CPelectrode accumulates individual lanthanides or a mixture with equalefficiency.

Interestingly, for Sc, Ce and Th the HIBA-CP electrode did notpre-concentrate above LOD. A possible explanation for the case of Sc isthat while in the same group as La, Sc responds in solution more as ad-element while Y, which does pre-concentrate, responds more like anf-element.

To gain some insight into the mechanism of HIBA-CP pre-concentration, acomparison of stepwise formation constants (log K) values for HIBA in0.1M ionic strength from Martell and Smith²⁹ and the total amount off-element pre-concentrated by the HIBA-CP electrode was conducted. SinceHIBA exhibits a systematic increase in log K values across the series oflanthanides,³⁷ one would expect that if HIBA-CP pre-concentrationcapability was solely a function of HIBA in the electrode, then asimilar trend would be observed. While in general, heavier lanthanidespre-concentrated more readily than lighter lanthanides, no directcomparison could be made indicating that more factors are involved inHIBA-CP pre-concentration capability and further work is required toelucidate these factors.

While this work has been performed in a neat solution, interferences areexpected since HIBA complexes to some extent with most metal cationspresent in solution. While many factors affect the strength ofmetal-ligand complexes, a good first approximation for determiningpotential interferences are thermodynamic stability constants. Nash andJensen thoroughly discuss the solution chemistry aspects of metal-ligandcomplexes and provide an excellent justification for the use ofstability constants for initial approximations. Surprisingly, while HIBAhas been in use since its first reporting in 1956, relatively littlecritically reviewed thermodynamic stability constant data are availablefor metal cations other than the f-elements.²⁹ While no stabilityconstant data exists for a Li⁺- HIBA complex, taking considerations ofionic charge, radius, and strength of ion-dipole interactions, weestimate that Li⁺ interactions with HIBA are minimal, resulting inlittle interference. In our case, Li⁺ was in 100,000-fold excess oftrivalent f-elements and did not serve as a major interference.

The experiments above shows that an MCP electrode can be used toselectively bind f-elements. In the preferred embodiment, the modifyingagent is a chemical agent with a selective binding affinity forf-elements. These modifying agents are included in the CP at a level ofat least 0.1 mmol/L (e.g. 10 milligrams of HIBA per liter of paraffinoil) but less than or equal to 20 mmol/L (e.g. 2,000 milligrams of HIBAper liter of paraffin oil).

These modifying agents are included in the CP at a level of at least 0.1mol/L but less than or equal to 20 mol/L (e.g. less than or equal to 10mol/ L).

Combination of the f-element concentration methods described herein withthe lanthanide separation methods described in Clark et. al. [Journal ofRadioanalytical and Nuclear Chemistry Volume 282 Issue 2 Pages 329-3332009], which is herein incorporated by reference, provides means forconcentration and separation of f-elements from industrial and/orenvironmental samples.

REFERENCES

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1-8. (canceled)
 9. A retrieving and depositing method for retrieving oneor more lanthanides or actinides from a first solution and depositingsaid one or more lanthanides or actinides in a second solution,comprising the steps of: contacting a first solution which contains oneor more lanthanides or actinides with a modified carbon paste electrodecomprised of an electrically conductive substrate with a carbon pasteadhered to a surface Surfaces of said substrate and with one ormodifying agents that selectively bind lanthanides or actinidesdispersed within said carbon paste; applying a first voltage to themodified carbon paste electrode suitable to deposit said one or morelanthanides or actinides contained in said sample onto the modifiedcarbon paste electrode; moving the modified carbon paste electrode to asecond solution; and applying a second voltage suitable to release ofsaid one or more lanthanides or actinides into said second sample. 10.The method of claim 9 wherein said first solution is an environmentalsample.
 11. The method of claim 9 wherein said first solution is anindustrial sample.
 12. The method of claim 9 wherein said secondsolution is of a smaller volume than said first solution, wherein saidsteps of contacting, applying, moving, and applying concentrate said oneor more lanthanides or actinides in said second solution.
 13. The methodof claim 9 wherein the modified paste electrode is repeatedly cycledbetween the first solution and said second solution, and said step ofapplying the first voltage is performed within the first solution andsaid step of applying the second voltage is performed within the secondsolution.
 14. The method of claim 9 wherein the one or more modifyingagents are selected from the group consisting of1,2-dihydroxybenzene-3,5-disulfonic acid, 2-hydroxyisobutyric acid,trimetaphosphoric acid, trans-1,2-cyclohexylenedinitrilotetraaceticacid, 2-hydroxy-2-methylpropanoic acid, iminodiacetic acid,nitrilotriacetic acid, ethylenedinitrilotetraacetic acid,diethylenetriamine-pentacetic acid,2,2′,2″,2′″-(1,4,7,10tetraazacyclododecane-1,4,7,10-tetrayl)tetraaceticacid, N-(2-carboxyphenyl)iminodiacetic acid, dihydroxycyclobutenedione,3,6,9,12-tetrakis(carboxymethyl)-3,6,9,12-tetraazatetradecane-1,14-dioicacid and derivatives of these compounds.
 15. The method of claim 14wherein the one or more modifying agents are reduced by said step ofapplying said first voltage and wherein said chemical agent is in aground state by said step of applying said second voltage.
 16. Themethod of claim 9 wherein the first voltage ranges from −0.05 to −0.05 Vand wherein the second voltage ranges from 0.1 to 0.09 V.
 17. A methodof selectively binding lanthanides and/or actinides in a sample,comprising: inserting in a sample containing one or more lanthanides oractinides a carbon paste electrode comprising an electrically conductivesubstrate, a carbon paste adhered to a surface of the substrate, and oneor more modifying agents dispersed within said carbon paste, wherein atleast one of said modifying agents includes an organic group and bindsto a plurality of lanthanide and/ or actinide elements; and applying avoltage to the carbon paste electrode sufficient to reduce the organicgroup of the one or more modifying agents from its electrochemicalground state or oxidized state.
 18. The method of claim 17 wherein saidone or more modifying agents are selected from the group consisting of1,2-dihydroxybenzene-3,5-disulfonic acid, 2-hydroxyisobutyric acid,trimetaphosphoric acid, trans-1,2-cyclohexylenedinitrilotetraaceticacid, 2-hydroxy-2-methylpropanoic acid, iminodiacetic acid,nitrilotriacetic acid, ethylenedinitrilotetraacetic acid,diethylenetriamine-pentacetic acid,2,2′,2″,2′″-(1,4,7,10tetraazacyclododecane-1,4,7,10-tetrayl)tetraaceticacid, N-(2-carboxyphenyl)iminodiacetic acid, dihydroxycyclobutenedione,3,6,9,12-tetrakis(carboxymethyl)-3,6,9,12-tetraazatetradecane-1,14-dioicacid and derivatives of these compounds, and wherein said applying stepapplies a negative voltage.
 19. The method of claim 17 wherein said oneor more modifying agents are selected from the group consisting of1,2-dihydroxybenzene-3,5-disulfonic acid, 2-hydroxyisobutyric acid,trimetaphosphoric acid, trans-1,2-cyclohexylenedinitrilotetraaceticacid, 2-hydroxy-2-methylpropanoic acid, nitrilotriacetic acid,diethylenetriamine-pentacetic acid,2,2′,2″,2′″-(1,4,7,10tetraazacyclododecane-1,4,7,10-tetrayl)tetraaceticacid, N-(2-carboxyphenyl)iminodiacetic acid, dihydroxycyclobutenedione,3,6,9,12-tetrakis(carboxymethyl)-3,6,9,12-tetraazatetradecane-1,14-dioicacid and derivatives of these compounds, and wherein said applying stepapplies a negative voltage.
 20. A system for selectively bindinglanthanides and/or actinides in a sample, comprising: a carbon pasteelectrode configured to be inertable in a sample containing one or morelanthanides or actinides, the carbon paste electrode comprising anelectrically conductive substrate, a carbon paste adhered to a surfaceof the substrate, and one or more modifying agents dispersed within saidcarbon paste, wherein at least one of said modifying agents includes anorganic group and binds to a plurality of lanthanide and/ or actinideelements; and a voltage source electrically connected to the carbon pastelectrode configured for applying a voltage to the carbon pasteelectrode sufficient to reduce the organic group of the one or moremodifying agents from its electrochemical ground state or oxidizedstate.
 21. The system of claim 20 wherein said one or more modifyingagents are selected from the group consisting of1,2-dihydroxybenzene-3,5-disulfonic acid, 2-hydroxyisobutyric acid,trimetaphosphoric acid, trans-1,2-cyclohexylenedinitrilotetraaceticacid, 2-hydroxy-2-methylpropanoic acid, iminodiacetic acid,nitrilotriacetic acid, ethylenedinitrilotetraacetic acid,diethylenetriamine-pentacetic acid,2,2′,2″,2′″-(1,4,7,10tetraazacyclododecane-1,4,7,10-tetrayl)tetraaceticacid, N-(2-carboxyphenyl)iminodiacetic acid, dihydroxycyclobutenedione,3,6,9,12-tetrakis(carboxymethyl)-3,6,9,12-tetraazatetradecane-1,14-dioicacid and derivatives of these compounds, and wherein the voltage sourceapplies a negative voltage.
 22. The system of claim 20 wherein said oneor more modifying agents are selected from the group consisting of1,2-dihydroxybenzene-3,5-disulfonic acid, 2-hydroxyisobutyric acid,trimetaphosphoric acid, trans-1,2-cyclohexylenedinitrilotetraaceticacid, 2-hydroxy-2-methylpropanoic acid, nitrilotriacetic acid,diethylenetriamine-pentacetic acid,2,2′,2″,2′″-(1,4,7,10tetraazacyclododecane-1,4,7,10-tetrayl)tetraaceticacid, N-(2-carboxyphenyl)iminodiacetic acid, dihydroxycyclobutenedione,3,6,9,12-tetrakis(carboxymethyl)-3,6,9,12-tetraazatetradecane-1,14-dioicacid and derivatives of these compounds, and wherein the voltage sourceapplies a negative voltage.